Carotenoids

Carotenoids 

(A group of highly specialized tetraterpenoids)

Introduction

These are fat soluble or yellow pigments present in plants & animals including bacteria these may be present along with chlorophyll (carotene & lutein) or may be present without chlorophyll. The carotenoid acts as photo sensitizer in conjugation with chlorophyll. When chlorophyll is absent (Fungi), then the carotenoids are mainly responsible for color. Carotenoids are also known as Lipochromes or chromo lipids because they are fat soluble pigments. They give a deep blue color with conc. H₂SO₄ & with a chloroform solution of antimony trichloride this Carr-Price reaction is the basis of the one method of the quantitative estimation of carotenoids. Some carotenoids are hydrocarbons, these are known as carotenes. Other carotenoids are oxygenated derivatives of the carotenes these are xanthophylls. These are also xanthophyll esters which are the natural ester of hydroxy carotenoids.

                                 Examples carotenoid & xanthophyll pigments from plants

Finally, there are some natural polyenes which contain fewer than 40 carbon atoms but structurally related to carotenoids. These are generally classified as the apocarotenoids & contains aldehyde or carboxyl group e.g. bixin (Annatto), Crocetin. When the loss of carbon atoms occurs at one end of the C₄₀ chain, this is shown by a numeral which follows the prefix apo & indicates the last carbon atom remaining from the present carotenoids e.g. β-apo-12 carotenal.

Carotenoid in fungi

These are tetraterpenoids main structure shows highly branched carbon skeleton composed of isoprene units, the center of molecule is formed by linkage of two isoprene unit tail to tail while others are head to tail. Hence central portion of molecule has 1, 6   position of methyl groups. Structurally these are polyenes having long conjugated chain in center of molecule. This portion which has extended conjugation is responsible for color of carotenoids of at the two ends are present two open chain structure or one open chain & other ring or two ring structure. Hence the basic structure of carotenoids becomes

Carotenoids are also known as Lipochromes or chromo lipids because they are fat soluble pigments. Carotenoids comprise of an important & unique class of carotenoids (C-40 terpenoids) in which two methyl group nearest the center of molecule are in position 1:6 while all the  other side chain methyl group occupies 1:5 position. Due to present of 3 isoprene units in structure they may also be regarded as tetra terpenoid.